Financing - Debt (Details) - USD ($) $ in Millions				6 Months Ended
		May 31, 2019	Jun. 01, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term debt and finance lease obligation				$ 3,618.8		$ 3,701.3
Repayments of Unsecured Debt				0.0	$ 185.0
Unsecured Debt
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 3,600.0		$ 3,100.0
Debt, Weighted-Average Interest Rate				5.14%		5.17%
Debt Instrument, Covenant Compliance				As of June 30, 2019, Boardwalk Pipelines and its operating subsidiaries were in compliance with their debt covenants.
Unsecured Debt | Boardwalk Pipelines 4.8% Notes Due 2029
Debt Instrument [Line Items]
Debt Instrument, Issuance Date		May 31, 2019
Debt Instrument, Face Amount		$ 500.0
Payments of Debt Issuance Costs		4.8
Proceeds from Debt, Net of Issuance Costs	[1]	$ 495.2
Debt Instrument, Interest Rate, Stated Percentage		4.80%
Debt Instrument, Maturity Date		May 03, 2029
Unsecured Debt | Boardwalk Pipelines Notes Due 2019
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 350.0
Debt Instrument, Interest Rate, Stated Percentage				5.75%
Unsecured Debt | Boardwalk Pipelines Notes Due 2018
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			5.20%
Repayments of Unsecured Debt			$ 185.0
Line of Credit
Debt Instrument [Line Items]
Debt, Weighted-Average Interest Rate						3.69%
Line of Credit Facility [Abstract]
Long-term Line of Credit				$ 0.0		$ 580.0
Line of Credit Facility, Covenant Compliance				The Company and its subsidiaries were in compliance with all covenant requirements under the revolving credit facility as of June 30, 2019.
Line of Credit | Amendment No. 1 Credit Agreement - 2016
Line of Credit Facility [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity				$ 1,500.0
Line of Credit | Amendment No. 2 Credit Agreement - 2017
Line of Credit Facility [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity				$ 1,475.0

[1] The net proceeds of this offering will be used to retire the outstanding $350.0 million aggregate principal amount of Boardwalk Pipelines 5.75% notes due 2019 (Boardwalk Pipelines 2019 Notes). Initially, the Company used the net proceeds to reduce outstanding borrowings under its revolving credit facility. The Company expects to retire all of the outstanding aggregate principal amount of Boardwalk Pipelines 2019 Notes at maturity with borrowings under its revolving credit facility.